1. Field of the Invention
The present invention relates to a differential switching circuit, more particularly a differential switching circuit comprising a pair of transistors having commonly coupled emitters used for a digital to-analog (D/A) convertor.
2. Description of the Prior Art
Prior art differential switching circuits, composed of a pair of transistors having commonly coupled emitters with a fixed current source connected to the emitters, suffer from the defect of an unstable output potential due to temperature-based variations of the current amplification. Additionally, there is the problem, in D/A converters etc., using a plurality of such differential switching circuits, of variation of current amplification factors hFE due to manufacturing differences in the elements in the circuits.
A conventional technique used to overcome the fluctuations, in a circuit used for a D/A converter with a resolution above 12 bits has been to provide an additional transistor for each transistor in the pair of transistors in a so-called "Darlington connection." The Darlington Connections, however, reduce the operating speed of the circuit. To raise the speed, a resistor has been inserted between the two transistors in each of the Darlington connections. However, this results in current fluctuation through the resistor.
In summary, the prior art differential switching circuit has had the defect of reduced operating speed when there is stable current amplification, and unstable current amplification when the operating speed is increased.